1. Field of the Invention
The present invention relates to a developing apparatus for forming a developer image by adhering developer to an electrostatic image formed on an image bearing body, which is suitable to be used with an image forming apparatus of electrophotographic type such as a copying machine, a printer, a facsimile and the like.
2. Related Background Art
As a developing apparatus used with an image forming apparatus of an electrophotographic type, various developing apparatuses have been proposed. For example, recently, there has been proposed a so-called nonmagnetic one-component DC contact developing system in which developing is effected by urging a semiconductive developing roller or a developing roller having a surface constituted by a dielectric layer against a surface layer of a photosensitive member.
FIG. 10 is a schematic constructural view showing an image forming apparatus (laser beam printer of an electrophotographic type) having a contact type one-component developing apparatus (referred to merely as xe2x80x9cdeveloping apparatusxe2x80x9d hereinafter) for effecting development by a conventional nonmagnetic one-component DC contact developing system.
The image forming apparatus includes a drum type electrophotographic photosensitive member (referred to as photosensitive drumxe2x80x9d hereinafter) 1 as an image bearing body. Around the photosensitive drum 1, there are disposed a charging roller 2, a developing apparatus 3, a transfer roller 4 and a cleaning blade 5, and an exposing apparatus 6 is disposed between the charging roller 2 and the developing apparatus 3. Further, a fixing apparatus is disposed at a downstream side of a transfer nip between the photosensitive drum 1 and the transfer roller 4 in a transfer material conveying direction.
The photosensitive drum 1 is for example an organic photosensitive member having negative charging polarity and is constituted by coating a photosensitive layer (not shown) on an aluminium drum substrate (not shown) and is rotated at a predetermined peripheral speed in a direction shown by the arrow (clockwise direction). During the rotation, the drum is negatively charged uniformly with predetermined potential.
The charging roller 2 as charging means is rotatingly contacted with the surface of the photosensitive drum 1 and serves to negatively charge the photosensitive drum 1 with the predetermined potential by charging bias applied to a charging bias power source (not shown).
The developing apparatus 3 is a contact one-component developing apparatus for effecting development with nonmagnetic toner T as one-component developer and includes a developing roller 9 as developer carrying body) opposed to the photosensitive drum 1 at an opening portion of a developing container 8 and is rotatable in a direction shown by the arrow (counterclockwise direction), a rotatable elastic roller 10 urged against the developing roller 9, a regulating blade 11 having elasticity and abutting against the developing roller 9, and an agitating member 12 for agitating the toner T in the developing container 8. The regulating blade 11 abuts against the developing roller 9 at a downstream side of the abut portion between the developing roller 9 and the elastic roller 10 in a rotational direction of the developing roller 9.
The toner T agitated by the agitating member 12 is supplied to a surface of the developing roller 9 by the rotating elastic roller 10 urged against the developing roller 9. The toner T supplied to the surface of the developing roller 9 is carried by the rotation of the developing roller 9 and is subjected to charging by friction in the abut portion between the regulating blade 11 and the developing roller 9 and is formed as a thin toner on the surface of the developing roller 9. The toner in the thin layer is carried by the rotation of the developing roller 9 and is adhered to the electrostatic latent image formed on the photosensitive drum 1 in the abut portion (developing portion) between the photosensitive drum 1 and the developing roller, thereby visualizing the latent image as a toner image. Incidentally, the toner on the developing roller 9, which did not contribute to the developing is scraped off by the elastic roller 10.
The transfer roller 4 as transferring means is urged against the surface of the photosensitive drum 1 with predetermined pressure to define a transfer nip therebetween and serves to transfer the toner image formed on the surface of the photosensitive drum 1 onto a transfer material P in the transfer nip between the photosensitive drum 1 and the transfer roller 4, by the transfer bias applied from the transfer bias power source (not shown).
The cleaning blade 5 serves to remove residual toner remaining on the surface of the photosensitive drum 1 after the transferring.
The exposing apparatus 6 includes a laser driver, a laser diode and a polygon mirror 14. In the exposing apparatus, a laser beam modulated in correspondence to a time-lapse electrical digital image signal of image information inputted to the laser driver is outputted from the laser diode, and, by scanning the laser beam by the polygon mirror rotating at a high speed and by effecting image exposure L on the surface of the photosensitive drum 1 through an optical lens system 15, an electrostatic latent image corresponding to the image information is formed.
The fixing apparatus 7 includes a fixing roller 7a and a pressing roller 7b. In the fixing apparatus, while the transfer material P is being pinched and conveyed in a fixing nip between the fixing roller 7a and the pressing roller 7b, the toner image transferred to the surface of the transfer material P is thermally fixed onto the transfer material by heat and pressure.
Next, an image forming operation of the image forming apparatus will be explained.
During the image formation, the photosensitive drum 1 is rotated by driving means (not shown) at the predetermined peripheral speed in the direction shown by the arrow and is uniformly charged by the charging roller 2. The image exposure L is given to the charged surface of the photosensitive drum 1 by the exposing apparatus 6, thereby forming the electrostatic latent image corresponding to the inputted image information.
By applying developing bias having the same polarity as a charging polarity (negative polarity) of the photosensitive drum 1 to the developing roller 9 of the developing apparatus 3, in the developing portion, the toner T charged with the same polarity as the charging polarity (negative polarity) of the photosensitive drum 1 is adhered to the electrostatic latent image formed on the photosensitive drum 1, thereby visualizing the latent image as the toner image. When the toner image on the photosensitive drum 1 reaches the transfer nip between the photosensitive drum 1 and the transfer roller 4, in synchronism with this, the transfer material P, such as paper supplied by a pick-up roller 16, is conveyed to the transfer nip through a registration roller pair (not shown).
By applying transfer bias having polarity (positive polarity) opposite to the polarity of the toner to the transfer roller 4, the toner image on the photosensitive drum 1 is transferred onto the transfer material P conveyed to the transfer nip. The transfer material P to which the toner image was transferred is conveyed to the fixing apparatus 7, where the transfer material P is heated and pressurized in the fixing nip between the fixing roller 7a and the pressing roller 7b to thermally fix the toner image to the transfer material. Thereafter, the transfer material is discharged onto a sheet discharge tray 17. In this way, a series of image forming operations are finished.
On the other hand, after the toner image transferring, residual toner remaining on the surface of the photosensitive drum 1 is removed by the cleaning blade 5 and is collected into a waste toner container 13.
Incidentally, in the image forming apparatus having the above-mentioned conventional nonmagnetic one-component contact type developing apparatus, as the number of imaged copies is increased, deterioration of the toner is prompted and the toner particles charged with polarity opposite to the normal polarity are increased. The toner charged with opposite polarity is shifted to a nonimaged portion on the photosensitive drum 1 (this phenomenon is referred to as xe2x80x9creversal fogxe2x80x9d). Although such reversal fog is hard to be transferred to the transfer material P and does almost not affect an influence upon the image, it increases toner consumption greatly. Consequently, the number of image formable sheets corresponding to a toner filling amount replenished in the developing container 8 is reduced, with the result that a service life of the developing apparatus cannot be extended.
Further, in the development of the conventional developing apparatus 3 of nonmagnetic one-component contact developing type, as shown in FIG. 11, so long as the charging amount of toner is in a proper range (|35| to |80| xcexcC/g), control can be effected in such a manner that a height of a line image becomes equal to a height of a solid image, and the line image can be controlled to required minimum toner height. Incidentally, in FIG. 11, the ordinate indicates a toner height ratio of line image/solid image, and the abscissa indicates the toner charging amount on the developing roller 9.
As the reason why the toner height of the line image becomes great, it is considered that, if the developing is permitted in an area (where the developing roller 9 is separated from the photosensitive drum 1) downstream of the abut portion (developing portion) between the developing roller 9 and the photosensitive drum 1, the toner on the developing roller 9 is swept together to increase the toner height of the line image. Further, if the toner charging amount on the developing roller 9 is increased, the toner holding force of the developing roller 9 due to a mirror symmetry force is increased, thereby preventing xe2x80x9csweeping-togetherxe2x80x9d. Thus, sharp lines and/or dots without scattering can be reproduced.
However, if the toner charging amount on the developing roller 9 is decreased due to the deterioration of toner caused by the increase in the number of imaged copies, the toner height of the line image is increased, with the result that not only the toner consumption is increased but also reduction of image quality such as line scattering or dot squeezing occurs.
In consideration of this, recently, in place of the above-mentioned nomnagnetic one-component contact developing system, for the purpose of obtaining stability of the toner charging amount and reduction in reversal fog, there has been proposed a developing system in which toner is electrically charged by using a toner charging roller as developer charging member.
A developing apparatus using the toner charging roller is shown in FIG. 12. Incidentally, elements having the same functions as those in the developing apparatus of the above-mentioned image forming apparatus are designated by the same reference characters and a duplicated explanation thereof will be omitted.
The illustrated developing apparatus 3 includes a rotatable toner charging roller 20 urged against the developing roller 9. The toner charging roller 20 is disposed at an upstream side of the regulating blade 11 in the rotational direction of the developing roller 9. The other arrangements are the same as those in the above-mentioned developing apparatus 3.
In the developing operation of this developing apparatus 3, the toner T agitated by the agitating member 12 is supplied to the surface of the developing roller 9 by the rotating elastic roller 10 urged against the developing roller 9. The toner supplied to the surface of the developing roller 9 to which the developing bias was applied is carried by the rotation of the developing roller 9 and is charged by friction in the abut portion between the regulating blade 11 and the developing roller 9 and is formed as a thin toner layer on the developing roller 9. Further, by electrical charging due to discharging of the toner charging roller 20 to which the charging bias was applied, charges are given to the toner on the developing roller 9. The toner formed as the thin layer and charged is carried by the rotation of the developing roller 9 and is adhered to the electrostatic latent image formed on the photosensitive drum 1 in the abut portion (developing portion) between the photosensitive drum 1 and the developing roller, thereby visualizing the latent image. Thereafter, the residual toner remaining on the developing roller 9, which was not developed in the abut portion between the photosensitive drum 1 and the developing roller 9 is removed by the elastic roller 10 and is returned to the developing container 8.
In this way, when the toner charging roller 20 is used, the reversal fog can be reduced and stable toner charging amount can be obtained.
However, in the developing apparatus 3 having the above-mentioned toner charging roller 20, as the number of imaged copies is increased, the toner charging roller 20 is contaminated by toner (particularly, toner having opposite polarity) to cause poor charging, thereby reducing the charge applying ability of the toner charging roller 20. Thus, the toner charging amount on the developing roller 9 is decreased and the reversal fog is increased.
An object of the present invention is to provide a developing apparatus which can obtain a stable toner charging amount even if a developer charging member is contaminated by toner and reduce reversal fog to reduce toner consumption.